The invention relates to the field of biasing of magnetic circuits, and more particularly to a technique for biasing the magnetic circuit of a magnetic recording head or a Hall-effect device using signals having noise-like properties.
In the magnetic recording art, it is common to apply high frequency (e.g. 100 KHz) AC biasing signals to the magnetic circuit of a magnetic recording head, such as used in a magnetic tape recorder. Such AC biasing is utilized to correct for distortion inherent in most magnetic systems. This distortion is due to the well known phenomena that the graph of magnetic induction (B) vs. magnetic force (H) in the gap of a magnetic circuit (such as a broken toroid of ferromagnetic material used in magnetic recording heads or in conjunction with Hall-effect devices) is not a smooth curve or straight line, but is a rather non-linear function. Thus the smooth periodic nature of a signal being applied to the magnetic circuit (e.g. an audio, video, or data signal), in the case of a magnetic recorder, or the measurement of electric current in the case of a Hall-effect device) will be distorted by the characteristic non-linear function of the magnetic circuit to produce an output which is a distorted version of the input signal. In the case of magnetic recording, the magnetic recording material also exhibits a highly nonlinear magnetic response to an external magnetic field. This also acts to distort a signal which is being recorded on the magnetic material. By applying AC biasing signals to the magnetic circuit, and hence to the magnetic recording material, the envelope of the resultant recorded signal will be shifted away from the highly non-linear portion of the B-H curve to a portion of the curve which is more nearly linear. This results in a composite signal being recorded which more accurately represents the signal being applied to the magnetic circuit. The application of AC biasing signals to the magnetic circuit of a Hall-effect device causes a similar improvement in the linearity of response of the magnetic circuit to an applied signal.
Such an AC biasing technique is well-known for use in the magnetic tape recorder art. See for example "Magnetic Recording", C. Lowman, McGraww-Hill, 1972, pp. 69-74. In addition, in Applicant's co-pending application Ser. No. 451,306, filed Dec. 20, 1982, entitled "Power Distribution in an Electrical Distribution System Having Three or More Wires", there is described a technique for utilizing an AC biasing system with the magnetic circuit of a Hall-effect device, similar to that described above.
One problem associated with the use of AC biasing in these prior art recording and power measuring systems is that the AC biasing frequency must be carefully selected to not be a harmonic or sub-harmonic of any of the normally expected frequencies of the signals of interest. In an audio recorder, the AC biasing signals are frequently placed several octaves higher than the highest expected frequency of the signals of interest (e.g. 20 KHz). Since the recording head is usually optimized for frequencies below 20 KHz, strong "roll-off" occurs above this frequency. Thus the AC biasing signals (which may have a frequency of around 100 KHz) will have to be injected at a fairly high amplitude into the magnetic circuit to insure adequate compensation for magnetic non-linearities. A similar approach is often used in data recorders, with the biasing frequency being chosen so as to minimize interference with the signals being recorded.
In a Hall-effect device used as a power measuring instrument, the AC biasing frequency is chosen to be several times higher than the line frequency of the electrical distribution circuit (e.g. 50 or 60 Hertz) and normally at a frequency which is not a harmonic of the power line frequency. This is to minimize the amount of correlation between the biasing signals and the measured power line signal, which might otherwise interfere with each other.
However, in both the magnetic recording and power measuring environments, the high frequency, high amplitude signals normally utilized for AC biasing can propagate as free electromagnetic waves or can be conducted over adjacent wires and circuitry and cause interference to the recording apparatus or power measuring apparatus or other peripheral equipment. In addition, since both the AC biasing signals and the signals being recorded or measured are both of a generally periodic (e.g. sinsuoidal) nature, there will always be some degree of correlation inherent between the signals, so that some error of offset in the recorded or measured signals may occur. Obviously, such error and offset signals can cause distortion in recording a signal or an error in the amount of power being measured by a Hall-effect power measuring device.